1. Field of the Invention
The present invention relates to electromagnetic couplers suitable for use in wireless communication systems to transfer information between information communication devices disposed at a short distance from each other via an electrostatic field or an induced electric field. The present invention also relates to information communication devices equipped with the electromagnetic couplers.
2. Description of Related Art
Conventional electromagnetic couplers include one disclosed in JP-B 4345851. This electromagnetic coupler (high-frequency coupler) is formed by connecting a plate-like electrode to a series inductor and a parallel inductor via a high-frequency transmission line. Such an electromagnetic coupler is to be disposed in an information communication device, such as a transmitter and a receiver. In the cases where a transmitter and a receiver are disposed so that the electrodes of their electromagnetic couplers face each other, when the distance between the two electrodes is 2λ/15 or smaller of the wavelength λ of the frequency used, the two electrodes are coupled by the electrostatic field component of longitudinal waves to behave as one capacitance and like a band pass filter as a whole, making it possible to efficiently communicate information between the two electromagnetic couplers. Also, when the distance between the two electrodes is in the range from 2λ/15 to 8λ/15 of the wavelength λ of the frequency used, information can be transferred by using an induced electric field of longitudinal waves.
Meanwhile, when the distance between the electromagnetic couplers is greater than a certain value, information cannot be transferred. As a result, other wireless devices do not suffer interference from electromagnetic waves generated from the electromagnetic couplers, and a wireless communication system in which information communication devices equipped with the electromagnetic couplers are used does not suffer interference from other wireless communication systems. Because of these characteristics, wireless communication systems in which the conventional electromagnetic coupler is used make it possible, by using an electrostatic field or an induced electric field of longitudinal waves over a short distance, to communicate a large volume of data between information communication devices by using the UWB (Ultra Wide Band) communication system, in which wide band signals are used.
More specifically, in the electromagnetic coupler described in JP-B 4345851, a through hole formed in a cylindrical dielectric is filled with a conductor, and a conductor pattern to be used as an electrode is formed on the top surface of the cylindrical dielectric. This cylindrical dielectric is mounted on a printed circuit board provided with a conductor pattern to be used as a high frequency transmission line, which is connected to the electrode via the conductor inside the through hole. The conductor inside the through hole substitutes for the above-mentioned series inductor, and the high frequency transmission line and a ground pattern are connected via a parallel inductor. When this electromagnetic coupler is fed with power, longitudinal waves of an electric field occur in the direction parallel to the conductor inside the through hole (the electric current flowing through the conductor inside the through hole). By using the longitudinal waves, information can be transferred.
Electromagnetic couplers are built into personal computers, cellular phones, digital cameras, etc. to transmit and receive data including moving images. Since electromagnetic couplers are built into small devices such as cellular phones and digital cameras, there is a strong demand for thinner electromagnetic couplers.
In the case of the electromagnetic coupler described in JP-B 4345851, however, the cylindrical dielectric needs to be shortened in order to obtain a thinner electromagnetic coupler. Then, the conductor inside the through hole becomes shorter, which means the electric field it generates becomes smaller. As a result, the longitudinal waves of the electric field also become smaller, causing a problem of a lower coupling strength between an electromagnetic coupler on the transmitter side and another electromagnetic coupler on the receiver side.
This lower coupling strength between the electromagnetic coupler on the transmitter side and the electromagnetic coupler on the receiver side means that information cannot be transferred when the distance between the two couplers is long. Also, even a small shift in the positions of the two electromagnetic couplers can disrupt the transmission of information, thus narrowing the coupling range.
In addition, in the electromagnetic coupler described in JP-B 4345851, a thinner electromagnetic coupler means its electrode approaches the ground. As a result, the impedance characteristics (characteristics of impedance with respect to frequency) become steep, while the input impedance of its feeding system is kept constant. This causes a problem of a narrower available frequency band (namely, a frequency band in which the matching condition between an electromagnetic coupler and its feeding system is favorable).
Also in the electromagnetic coupler described in JP-B 4345851, when the distance between the electrodes of two electromagnetic couplers is 2λ/15 or smaller of the wavelength λ of the frequency used, information can be efficiently communicated between the electromagnetic couplers by forming a band pass filter. Unfortunately, however, when the match between the two electromagnetic couplers is not favorable, signal transmission efficiency is degraded.
On the other hand, in the case of wireless communications carried out by using devices provided with this electromagnetic coupler described in JP-B 4345851, for example, a cover or a case of each device including a dielectric exists between the electromagnetic couplers, resulting in variations in the dielectric constant between the electromagnetic couplers. Then, variations occur in the value of the capacitance between the electrodes of the two electromagnetic couplers and in the frequency characteristics of the band pass filter, which in some cases may degrade the information transmission characteristics in the frequency band of interest. Even if these expected variations in the dielectric constant in some cases are taken into account in designing the electromagnetic couplers, in the case of wireless communications carried out by using other devices made of different materials and/or differently designed, the value of the dielectric constant between the electromagnetic couplers varies, which similarly degrades the information transmission characteristics in the frequency band of interest.
Also, in the electromagnetic coupler described in JP-B 4345851, when the distance between the electrodes of the two electromagnetic couplers is in the range from 2λ/15 to 8λ/15 of the wavelength λ of the frequency used, information is communicated by using the induced electric field component of longitudinal waves. In this case, when the relative position of the two electromagnetic couplers and the environment are kept constant, the information transmission characteristics depend on matching conditions between the electromagnetic couplers and the feeding system. In other words, the signal intensity from the electromagnetic couplers to the communication module including the feeding system increases under a favorable matching condition, while the signal intensity from the electromagnetic couplers to the communication module including the feeding system decreases under a poor matching condition.
Accordingly, in the electromagnetic coupler described in JP-B 4345851, electromagnetic couplers must be designed so that a band pass filter is formed when the distance between the electromagnetic couplers (between the two electrodes) is 2λ/15 or smaller of the wavelength λ of the frequency used, and that the matching condition becomes favorable when the distance between the electromagnetic couplers is in the range from 2λ/15 to 8λ/15 of the wavelength λ of the frequency used. Therefore, in the case of an insufficient signal intensity when the distance between the electromagnetic couplers is in the range from 2λ/15 to 8λ/15 of the wavelength λ of the frequency used, for example, a redesign is required with a view to forming a band pass filter when the distance between the electromagnetic couplers is 2λ/15 or smaller of the wavelength λ of the frequency used. This means that much time and effort is required in designing the electromagnetic couplers. In addition, when the frequency band to be used is broad, it is required to obtain a large number of frequencies in which the matching condition is suitable, which means that even more time and effort is required.